CPU/GPU maxed out, playback super laggy

Your computer is in good shape but you have to know that the type of media you work with has a massive impact on playback performance. I didn't see attached video but I know that GoPro footage is going to be in a compressed/interframe codec like h264/h265, which is not good for editing. Add that it's 4k and high framerate and that's going to further increase the computer power required to decode. Add an effect or color grading and that's going to add to it even further (depending on the effect it can have a huge impact.) Do you get a red render bar with the effect added? That's basically telling you that you cannot expect real-time playback without rendering first.

Solution: transcode to an intermediate codec (read: good for editing) before working with it, or make proxies into a low-res/low-bitrate intermediate codec and just toggle the proxies on. For the most part I work with proxies (rather than transcoding before-hand) and I tend to use the ProRes Proxy preset. In my opinion you don't need to make your own custom ingest/encoding presets unless you need/want more control (like adding a watermark/timecode) or you're working with a strange aspect ratio -- and even then, ProRes is usually pretty adaptable. (I say this last part because if you look up a tutorial on making proxies, depending on what you find you might think you have to create a custom preset, but you don't, and it's probably unnecesarily complicated if you're new to proxies.)

Adding the screenshots here to make it easier